Hyperbolic magnet poles for sink-float separators

ABSTRACT

A hyperbolic magnet with pole which produces a constant magnetic gradient along an axis, usually a vertical axis, in a sink-float separator. Magnetic pieces are secured at the lateral edges of the poles to modify the magnetic field to prevent material being separated from being pinned to the outer walls of the separator and a mirror plate is adjustably separated from the end of the poles for tuning the magnetic field.

United States Patent Kaiser et al. Aug. 5, 1975 [5 1 HYPERBOLIC MAGNETPOLES FOR 2.768.746 10/1956 COlbUlTl 209/223 R SINK FLOAT SEPARATORS3,289,836 12/1966 Weston 209/222 X 3.483.969 12/1969 Rosensweig 209/1 [7In n r Robert Kaiser. m g Leon 3.733.465 1/1974 Reimers et a1 209/1 Mir,Brookline, both of Mass.

[73] Assignee: Avco Corporation, Cincinnati, Ohio [22] Filed: Mar. 25,1974 [21] App]. No: 454,373

[52] US. Cl 209/1; 209/1725 [51] Int. Cl. B03B 5/00 {58] Field of Search209/1, 172.5, 223 R, 232, 209/1118; 310/10 [56] References Cited UNITEDSTATES PATENTS 1,956,760 5/1934 Forrer 209/223 R 2,088,364 7/1937 Elliset a1. 209/232 X 2,154,010 4/1939 Queneau 209/232 X PrimaryExaminerFrank W. Lutter Assistant Examiner-Ralph J. Hill Attorney,Agent, or Firm-Charles M. Hogan, Esq.; Abraham Ogman, Esq.

[5 7] ABSTRACT A hyperbolic magnet with pole which produces a constantmagnetic gradient along an axis, usually a vertical axis, in asink-float separator. Magnetic pieces are secured at the lateral edgesof the poles to modify the magnetic field to prevent material beingseparated from being pinned to the outer walls of the separator and amirror plate is adjustably separated from the end of the poles fortuning the magnetic field.

8 Claims, 7 Drawing Figures PATENTEU WE SHEET PATENTEU AUG 51975 SHEETHYPERBOLIC MAGNET POLES FOR SINK-FLOAT SEPARATORS BACKGROUND OF THEINVENTION l. Field of the Invention The present invention relatesgenerally to sink-float separators and more particulary to a newmagnetic pole design for sink-float separators.

2. Description of the Prior Art In the sink-float separators, it hasbeen found common to use magnetic fields to cause the separation ofobjects of different densities in a ferrofluid. The magnetic fieldproduced in the prior art has not been suffcient to provide accurateseparation of objects whose densities are very close. Similarly, themagnetic fields of the prior art have not been sufficiently adjustableso as to fine tune the system to guarantee a constant apparent densityin a ferrofluid, hereinafter called magnetic density.

A problem that arises in sink-float tank separators of the prior art isthat the magnetic field is such that it results in the pinning of thematerials which are to be separated against the outer walls of theseparator, thereby jamming the free-flow in the vertical direction ofthe objects to be separated.

Also, prior art devices which produce acceptable magnetic fields uselarge amounts of electric power.

SUMMARY OF THE INVENTION The present invention is a unique design of amagnet with hyperbolic poles and a mirror plate for sink-floatseparators to solve the problems of the prior art devices. Therectangular hyperbolic magnetic pole pieces and the mirror plate aredesigned to produce a constant vertical magnetic gradient in the magnetgap. The pole pieces are straight-line approximations of a portion of ahyperbola, said portions asymptotes being parallel to the direction ofseparation or gravity. Magnetic pieces, or shims, are secured to thelateral edges of the poles to locally modify the magnetic field toprevent the material being separated from being pinned to the outerwalls of the separator by the force produced by the magnetic field ofthe poles. The magnetic plate is adjustably separated from the ends ofthe poles to fine tune the magnetic field produced by the hyperbolicpoles to provide a constant magnetic density throughout the ferrofluid.

OBJECTS OF THE INVENTION An object of the present invention is toprovide a magnetic pole design with the lower power requirements thanthe prior art devices.

Another object of the present invention is to provide a magnetic poledesigned for use in the float-sink separator which provides a uniformvertical magnetic field gradient.

A further object of the present invention is to provide a magnetic poledesign which is fine-tunable so as to achieve a uniform magnetic densityacross in a ferrofluid volume.

Still another object of the present invention is the modification of themagnetic field so as to prevent objects being separated from beingpinned against the outer walls of the separator.

Other objects, advantages and novel features of the present inventionwill become apparent from the following detailed description of theinvention when considered in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. I is a hyperbolic curve fromwhich the preferred embodiment of the magnetic poles were designed;

FIG. 2 is a front view of a float-sink separator with the preferredhyperbolic poles;

FIG. 3 is a front view of the gap defined by the hyperbolic poles andmirror plate of FIG. 2;

FIG. 4 is a view of the hyperbolic poles and mirror plate of FIG. 2taken along Line 44 of FIG. 3;

FIG. 5 is a top view of the preferred embodiment shown in FIG. 3,showing a modification of the shims;

FIG. 6 is a graph of the magnetic field H along the Z axis; and

FIG. 7 is a graph of magnetic parameters as a function of verticalheight before and after fine tuning using the adjustments of thepreferred embodiment.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The non-magnetic sink-floatseparator of the present invention is based on an unusual property offerrofluids the ability to float a non-magnetic or weakly magneticobject of far greater density than the ferrofluid itself when theferrofluid is placed in a suitable magnetic field.

Ferrofluids are very stable colloids of small (about A) magneticparticles. generally magnetite, suspended in a base liquid andstabilized by surface active agents. For sink-float separation, the baseliquid could be, for example, a high flash point kerosene. The suspendedparticles do not settle out or agglomerate under the action of gravityor applied magnetic fields. A ferrofluid placed in a non-uniformmagnetic field experiences a force directed along the magnetic fieldgradient. A consequence of this is that a non-magnetic object immersedin a ferrofluid experiences a force in the opposite sense and isexpelled to the region of mini mum magnetic field.

In a ferrofluid separator, a body of ferrofluid is held between thepoles of an electromagnet which generates a magnetic field with aconstant gradient; directed downward, in the direction of gravity.Consequently, a non-magnetic object immersed in the ferrofluid poolexperiences a magnetic force in the upward direction. By regulating thestrength of the magnetic field gradiem and the strength of theferrofluid, this magnetic force can be made larger or smaller than theforce of gravity on the non-magnetic object. When the magnetic force islarger than the force of gravity, the object will float even though itsdensity is larger than the density of the ferrofluid. When the magneticforce is less than the force of gravity, the object sinks.

According to the theory of ferrofluid levitation, in a suitable magneticfield with a gradient parallel to gravity, a ferrofluid can be viewed asa liquid that has a controllable apparent density:

M H p a p F 4 m where p a apparent density of the ferrofluid', g/cm p Fphysical density of ferrofluid; g/cn't M(H) magnetic dipole moment offerrofluid. emu;

a function of the field strength, H

1 vertical gradient of magnetic field; oersted/cm 3 acceleration ofgravity; cm/sec" The apparent density can be changed by changing whichis done by changing the current to the coil of the electromagnet. Withexisting fcrrofluids and state of the art electromagnet design, it ispossible to obtain apparent densities that range from about I g/cm toover 25 g/cm". This concept is more fully discussed in US. Pat. Nos.3,483,968; 3,483,969 and 3,488.531, which are commonly assigned to theassignee hereof and are incorporated herein by reference.

In order to obtain accurate separations, it is necessary for theapparent density of the ferrofluid to be constant (within the accuracydesired) throughout the ferrofluid volume. Otherwise, the less denseobjects might fall to the bottom in a region of an abnormally lowapparent density and the more dense objects float in a region ofabnormally high apparent density. This is accomplished by ensuring thatthe field throughout the separator is high enough to bring theferrofluid magnetic dipole moment M close to its saturation value andthat the vertical magnetic field gradientv" is essentially constant.

An additional constraint on the desired magnetic field is that thehorizontal gradients (V x and z) be very small, or that they beoutwardly directed. This constraint is required to prevent the levitatedobjects from moving toward the bounding walls of the separator as theyrise or fall. Were this to occur, the objects would be pinned againstthese walls and jamming would result. By designing the magnet to provideoutwardly pointing gradients, the particles move away from the walls tothe center of the ferrofluid pool.

Although it is possible to achieve such a design by the use of Helmholtzcoils, a design based on properly shaped iron pole pieces is morepractical because of its very much lower power requirements. The basicshape of such pole pieces is a rectangular hyperbolic surface extendingindefinitely in the +2 and z direction. A mirror plane plate is providedabove the pole pieces to create a virtual image of the pole pieces. Thepole is a straight line approximation of the solid segment of thehyperbola shown in FIG. 1.

For this configuration, the value of the vertical component of themagnetic field gradient is given by:

G, the value of the gradient along the y axis is controlled by thedistance between the poles and the magnetizing current. By limiting theferrofluid to region near the y axis, where the value of .r/y is small,the value of 8 H/lS y is kept as constant as the separation to be madedictates.

in translating this theoretical design to the hardware stage, the upperhalf of the hyperbolic surface (not shown) is replaced by a steel platecoincident with the x axis, as shown in FIG. 1. This plate forces thefield lines to be perpendicular to the x axis, and is thereforefunctionally equivalent below the x axis to the upper hyperbolicsurface. Three deviations to the shape of the lower hyperbolic surfaceare also introduce:

1. Since the gradient drops sharply for the large values of .r. theregion between the x axis and the portion of the curve near this axis isnot used for separation. The hyperbolic surface is therefore terminatedat some finite value of .r.

2. The hyperbolic surface is also terminated at a finite value ofy,because for large values of y. the interpole distance becomesimpractically small.

3. Likewise one cannot extend the poles indefinitely along the z axis.

The above cut-offs from the ideal hyperbolic shape necessitatecorrections to the shape of the pole piece in order to achieve the twobasic goals of the design given previously. The pole pieces shown inFIGS. 2, 3 and 4 were designed to include the necessary correction andachieve the aforementioned goals.

The preferred embodiment as shown in FIGS. 2, 3 and 4 includes theseparator 10, having included therein two hyperbolic segments 12 and 14.Each of the hyperbolic pole pieces 12 and 14 may be made up of aplurality of plane segments which approximate a portion of thehyperbolic surface as shown in FIG. 1. Similarly, the pole pieces may becast as a unitary piece and the face machined to form the planesegments.

The hyperbolic pole pieces 12 and 14 are supported and secured to a yoke16. Lying perpendicular to the y'axis on support blocks 18 is the mirrorplate 20 and mirror plate support 22 discussed above. Though the mirrorplate support block 18 is shown as blocks, they may be replaced by anydevice which will adjustably mount the mirror plate relative to the polepieces 12 and 14 so as to fine tune the system. As will be discussedlater, a system may be fine tuned so as to find the perfect distancefrom the mirror plate to the pole pieces 12 and 14.

Secured to both ends of the magnetic pole pieces 12 and 14 are aplurality of magnetic pieces or shims 24. Magnetic pieces 24 are used toprovide a magnetic field distribution as shown in FIG. 6. The shimsincrease the magnetic field at the marginal edges of the pole pieces togenerate the hill-like perturbations 25 at the edge of the pole pieces.The perturbation is provided to assist the material being separated toflow towards the center of the magnetic field. This action prevents theforce produced by the magnetic field from pinning the mate rial againstthe container wall.

The magnetic field gradients found in the perturbations are generally inthe direction shown by arrows 27 and 29. Materials to be separated whichgravitates to a boundary 31 of the working volume, encounters themagnetic field gradient depicted by arrow 27. This material is forcedback away from the boundary 31 by the outwardly directed lateralgradient 27 since, as discussed previously, the material tends to movetoward regions of minimum magnetic field.

There is no necessity to modify the magnetic field along the .r axis asthe pole pieces are designed to provide the dish-like configurationalong the x axis as shown for the z axis of FIG. 6. If modification ofthe magnetic field along the x axis is desired, as is provided along thez axis, the modification of the shims to provide such a fieldmodification is shown in FIG. 5 as 24' as being tapered in two planes.As can be seen clearly in FIG. 3, the shims 24 are rectangular and havea generally tapered configuration in one plane which may vary between /zto 7, dependent upon the size and structure of the magnetic pole pieces.

The working volume" marked in FIG. 3 is the region where the verticalcomponent of the gradient should be constant within This region isobtained by having the ideal hyperbolic shape of the poles approximatedby the straight line cords that intersect on the hyperbola and themirror plate. The tapered shims along the edges of the poles are for thepurpose of modifying the magnetic field and controlling the horizontalgradient.

In summary, without the shims, an inwardly directed gradient is producedby the pole piece. This gradient, along the z axis, exerts a force thatwill push the materials in the ferrofluid towards the front and rearwalls of the separator. The shims modify the field to produce the fieldas shown in FIG. 6. An outwardly pointed 2 component of the magneticfield gradient prevents the particles which are being separated frombeing pinned to the containers outer wall. The objects to be separatedmove along the y axis which, as can be seen in FIG. 1, is one of theasymptotes of the hyperbola, of which the magnetic poles are sections.

As shown in FIG. 2, two coils 26 and 28 are wrapped around the yoke 16behind pole pieces 12 and 14, respectively. It is to be understood thatthe magnetic field is of sufficient strength to support the ferrofluidtherebetween. A container 30, shown in FIG. 2, is included to restrictthe ferrofluid to the working volume and provide the front and rearwalls of the separation. When the top and bottom of box 30 are deleted,there is easy horizontal access to the pool of ferrofluid. Sincemagnetic forces also retain the ferrofluid in the gap of the magnet,conveyors or other means of introducing the feed or removing theseparated products can be introduced directly into the ferrofluid poolwithout fluid leakage or sealing problems from front or back.

It is generally well known to use a magnetic mirror plate to generate avirtual mirror image of the field distribution. The mirror plate of thepresent invention is used, however, to fine tune the present system. Asillustrated in FIG. 7, the mirror plate 20 is located in an optimumposition to provide a constant gradient (Curve 33), and the magneticmoment M, the field intensity H, and the density p depicted as Curves31, 32 and 37, respectively. As depicted in Curve 37, the density p isslightly less at the top of the magnetic pole than it is at the bottom.

To effectively separate objects having very close densities, it isimperative that the density p be constant. To achieve this, the magneticplate 20 may be varied to fine tune locally the system so as to achievea constant p. The height of the mirror plate 20 is determined by theheight of the support blocks 18. When the mirror plate is loweredslightly, there is a large change in the field intensity H, a slightdecrease in the magnetic moment M, and a relatively large increase inthe vertical gradient in the vicinity of the mirror plate 20. If themirror plate is lowered the proper amount, will increase in the vicinityof the mirror plate to provide a constant p throughout the workingvolume. The

6 parameters H. M. Wand p. as a function of height Y. after theadjustment. are represented as H. M. and p and are depicted by curves35. 36. 34 and 38. respectively.

To summarize briefly, the hyperbolic magnetic design as depicted inFIGS. 2, 3 and 4 provide a constant p. The mirror plate 20 is adjustableto increase in the vicinity of the mirror plate to a greater extent thanM is decreased, so that a constant p can be realized.

The magnetic field between the poles, a nominal 4" separator, weremapped by means of a Hall probe after energizing coils 26 using 10 amps.The results of such mapping is shown in Table I attached. It should benoted that the coordinate system x, y, and z differs as shown in FIGS.2, 3 and 4 from the coordinate system shown in FIG. I because the x, zaxes are offset along the y axis.

It is apparent that along the axis of the system (x' O, z 0), thegradient is very constant from y' 0.8 inches to y' 3.0 inches. Near thecomers of the working volume (x 1.0, z 1.0), the gradient near y 3.0 isabout 8% lower than the axis value. At intermediate points, thedeviations are correspondingly lower. This pole shape therefore meetsthe first objective of the design, a nearly constant vertical gradientover a substantial portion of the interpole volume.

The data in Table I shows that the .i" component of the gradient pointsaway from the center. towards the poles of the magnet. Within thecontrolled working volume, the z Component of the gradient is verysmall, less than 10% of the value of the y component. Near the innerportion, it points to the axis but near the outer portion, it pointsaway from the axis of the interpole volume. Thus, the second objectiveof the design is also met by this pole geometry.

Scaling up this design can be carried out directly without any change inpole shape, while still maintaining the desired field characteristics.It is, of course, necessary to provide the appropriate increases in theampere turns of magnetizing current, in order to achieve the samegradient values.

The present design can separate any two non-ferrous materials whosedensity differs by 5 to 10% or more. Apparent specific gravity of theferrofluid can bc varied, from less than 1 g/cm to over 20 g/cm by a 5mph: change in electric current flow through the coils of an energizingelectromagnet. This range include from magnesium and aluminum (around2-3 g/cm); through zinc, tin, brass, copper and lead (7-1 1 g/cm); up togold and platinum 1922 g/cm). Some magnetic materials may also beseparated as long as they are less magnetic than the ferrofluid used.

The separation of non-magnetic metals was carried out in a ferrofluidpool contained between the poles of this magnet. In all cases, theapparent ferrofluid density required to float an object was equal,within experimental error, to the objects known density, as thefollowing table demonstrates:

TABLE I FIELD MAPPING OF HY PERBOLIC POLES Position of Probe Position ofProbe Position of Probe Position of Probe Position of Probe x'=0.0",z'=0.0" x=0.8, z'=0.0 x'=-.8", z'=l .0 x'=l.0, z'=.0 x=l.0", z'=l.0 v IH ,1 H H H (inches) (k. gauss) (inches) (k. gauss) (inches) (k. gauss)(inches) (k. gauss) (inches) (k. gauss) TABLE I- Continued FIELD MAPPINGOF HYPERBOLIC POLES Position of Probe Position of Probe x'=0.0", z'=0.0"x=0.8. z'=().0 x'=-.8". z=1.0 x'=l.0 z'=.0 x'=l.0", z=l.() H H H H(inches) (k. gauss) (inches) (k. gauss) (inches) (k. gauss) (inches)(1:. gauss) (Inches) (k. gauss) Apparent Ferrofluid it is possible toaccurately separate materials differing Dens'w Densny in density by aslittle as the separation being es- Brass 8.3 8.4 sentially independentof the size or shape of the ob- Copper 8.9 8.7 jects Aluminum 2.7 2.8zinc 7 7 Although the invention has been described and Illus- Titanium4.5 4.6

The separation of mixtures of brass and copper occurred when theapparent ferrofluid density was adjusted to values intermediate between8.4 and 8.7. This was obtained with ferrofluids varying insaturationmagnetization from 100 gauss to 240 gauss.

The especially designed hyperbolic magnetic poles for use in asink-float separator achieves the desired design requirements andutilizes low power requirements.- The design provides a constantvertical magnetic gradient and includes the adjustment of the mirrorplate as defined in the system to provide a constant density along thevertical axis. Shim pieces are provided at the edges of the magneticpoles to modify the magnetic field so as to prevent the objects whichare being separated from being pinned against the outer walls of thecontainer.

The pole configuration of the present invention can be used in theprocess described in the previously mentioned patents. The solid mixtureto be separated is introduced into the pool of ferrofluids suspendedbetween the poles by magnetization. Objects less dense than the apparentdensity of the ferrofluid float to the top, while those more dense sinkto the bottom of the ferrofluid pool, resulting in a physicalseparation. An upper and lower conveyor will remove the more dense andless dense materials. respectively, from the top and bottom of the pool.Since magnetic forces also retain the ferrofluid in the gap of themagnet, conveyors can be introduced directly into the pool without fluidleakage or sealing problems.

The separation can be carried out on a batch or continuous basis.Mixtures of three or more components can be separated by making two ormore passes through the separator during which the apparent density ofthe ferrofluid is adjusted to produce one pure component per pass. Suchmixtures could also be separated by a sequence of separators operatingat appropriate apparent density levels, with partly separated mixturesbeing conveyed from one separator to the next until all separations areachieved. By such means,

trated in detail, it is to be clearly understood that the same is by wayof illustration and example only and is not to be taken by way oflimitation, the spirit and scope of the invention being limited only bythe terms of the appended claims. What is claimed is: l. [n a ferrofluidseparator having a magnet including a pair of spaced pole piecesdefining an air gap containing a magnetic field and a pool of ferrofluiddisposed in said air gap and magnetic field, the improvement comprising:

pole pieces that are a' mirror image of each other with respect to anaxis, and each of said pole pieces is a segment of a hypobolic surface;and

a mirror plate means disposed above and spaced from said pole pieces andair gap for creating a virtual image of said pole pieces.

2. The separator as in claim 1 wherein each pole is formed by aplurality of pole pieces. each being a straight line approximation of aportion of said hyperbolic segment.

3. The separator as in claim 2 including means secured to said poles forpreventing the materials to be separated from being pinned to thelateral walls of the ferrofluid pool.

4. The separator as in claim 3 wherein said means includes magneticpieces secured to the lateral edges of said poles.

5. The separator as in claim 4 wherein said magnetic pieces are taperedshims with a taper in one plane.

6. The separator as in claim 4 wherein said magnetic pieces are taperedshims with a taper in two planes.

7. The separator as in claim 1 to include means for maintaining aconstant apparent density along said axis which includes means foradjusting the magnetic field intensity adjacent to said mirror plate.

8. The separator as in claim 7 wherein said maintaining means includessaid magentic plate, whose plane is perpendicular to said axis and saidmaintaining means being adjustable along said axis for varying theapparent density distribution of the magnetic field.

1. I II UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION PATENT NO.3, 898,156 DATED g t 5, 1975 INVENT()R(S) 1 Robert Kaiser, Leon Mir Itis certified that error appears in the above-identified patent and thatsaid Letters Patent are hereby corrected as shown betow:

Column 3, line 6, please insert-- before "Which.

Column 3, line 68, please change "introduce" to "introduced".

Column 7, line 50, please change t0 Signed and Scaled this twenty-firstDay Of October 1975 [SEAL] Attest:

RUTH C. MASON C. MARSHALL DANN Arresting Officer (umnu'ssz'uneruj'larenrs and Trademarks

1. IN A FERROFLUID SEPARATOR HAVING A MAGNET INCLUDING A PAIR OF SPACEDPOLE PIECES DEFINING AN AIR GAP CONTAINING A MAGNETIC FIELD AND A POOLOF FERROFLUID DISPOSED IN SAID AIR GAP AND MAGNETIC FIELD, THEIMPROVEMENT COMPRISING: POLE PIECES THAT ARE A MIRROR IMAGE OF EACHOTHER WITH RESPECT TO AN AXIS, AND EACH OF SAID POLE PIECES IS SEGMENTOF A HYPOBOLIC SURFACE, AND A MIRROR PLATE MEANS DISPOSED ABOVE ANDSPACED FROM SAID POLE PIECES AND AIR GAP FOR CREATING A VIRTUAL IMAGE OFSAID POLE PIECES.
 2. The separator as in claim 1 wherein each pole isformed by a plurality of pole pieces, each being a straight lineapproximation of a portion of said hyperbolic segment.
 3. The separatoras in claim 2 including means secured to said poles for preventing thematerials to be separated from being pinned to the lateral walls of theferrofluid pool.
 4. The separator as in claim 3 wherein said meansincludes magnetic pieces secured to the lateral edges of said poles. 5.The separator as in claim 4 wherein said magnetic pieces are taperedshims with a taper in one plane.
 6. The separator as in claim 4 whereinsaid magnetic pieces are tapered shims with a taper in two planes. 7.The separator as in claim 1 to include means for maintaining a constantapparent density along said axis which includes means for adjusting themagnetic field intensity adjacent to said mirror plate.
 8. The separatoras in claim 7 wherein said maintaining means includes said magenticplate, whose plane is perpendicular to said axis and said maintainingmeans being adjustable along said axis for varying the apparent densitydistribution of the magnetic field.